1. Field of the Invention
The present invention relates generally to communication systems, and more particularly to the apparatus and method of error vector measurement (EVM) for the production line testing for general digital communication systems and the rate adaptation for multiple-rate communication systems.
2. Background Art
For communication systems, signal quality measurements are important for several reasons. One of the reasons is the production line testing (PLT). PLT is required to determine whether every component in a transmitter (Tx) and a receiver (Rx) works correctly. Good measured signal qualities with given conditions guarantee that PLT is passed for those conditions. Another reason is that this measured signal quality may be used as an indicator of the current physical channel condition including Tx and Rx distortions. In other words, by measuring the signal quality, the supported data rates for a wireless channel can be easily accessed.
Instant signal quality measurements are important especially for wireless communication systems that support multiple data rates. For example, wireless multiple-input multiple-output (MIMO) systems, such as IEEE 802.11n [1], naturally have large set of data rates. Due to the time and/or range variation of the wireless channels and a large set of data rates, it is important to find an optimum data rate on the current channel condition.
In IEEE 802.11n, the data rates are varied with the bandwidth, the number of spatial data streams and each stream's modulation and coding rate (MCS). Since the number of spatial data streams and each stream's MCS can be easily changed during connection, throughput may be optimized by adapting data rate with instantly measured signal quality feedback from the receiver. Rate adaptation is a mechanism to select an optimum data rate supported on the current channel condition to maximize the data throughput.
Received signal-to-noise ratio (SNR) is one of the possible indicators for decoded signal qualities. However, received SNR alone is not a good indicator especially for wireless multiple-input multiple output (MIMO) systems and most of the time can not be obtained easily at the receiver. Interference from the other spatial data streams may be also considered to determine the optimum transmit data rates. That is why signal-to-interference and noise ratio (SINR) may be a better indicator for MIMO systems. Unfortunately, it is not simple to calculate the received SNR or SINK because the received signal is the combination of the signal, the interference and the noise. The signal, the interference and the noise are hard to separate since they are unknown until correct decoding.
Because of this difficulty, in some rate adaptation algorithms, SNR (or SINR) is replaced with error vector measurement (EVM). EVM is the average distance between the received signal points and the closest constellation points. It is much simpler to measure EVM because the closest constellation points can be instantly determined with simple operations. The EVM and SNR (or SINK) are not identical, but closely related. That is, The EVM is also a good indicator for decoded signal qualities.
On the EVM, U.S. Pat. No. 20070041322 provided by to Choi et al., entitled “Rate adaptation using semi-open loop technique”, discloses that EVM is used to indicate the channel condition and transmitter characteristics and thus used for rate adaptation. U.S. Pat. No. 6,922,549 issued to Lyons et al., entitled “Error vector magnitude selection diversity metric for OFDM”, discloses EVM is used for antenna selection, and the preamble EVM used for intra-packet antenna selection is mentioned. U.S. Pat. No. 20050163252 provided by to McCallister et al., entitled “Transmitter predistortion circuit and method therefor”, discloses that EVM is used to calibrate the predistortion circuits of the transmitter. U.S. Pat. No. 20050163252 provided by to Sun et al., entitled “Multiple-input multiple output system and method”, discloses a MIMO system that used channel estimation and pilot EVM for feedback information in a closed-loop optimization. U.S. Pat. No. 20050163252 provided by to Walvis et al., entitled “Apparatus and method for simultaneous testing of multiple orthogonal frequency division multiplexed transmitters with single vector signal analyzer”, discloses a apparatus and method for testing signals from two or more orthogonal frequency division (OFDM) transmitters simultaneously with a single vector signal analyzer. However, the aforementioned disclosures uses expensive high-precision devices with pre-calibrated Rx units in transmit to achieve sufficient accuracy tests. According to the above problems, therefore it is needed to provide a low cost testing and Rx unit, which can save time since testing simultaneously multiple transmitters will be more affordable. In the present invention, a novel apparatus and method of error vector measurement (EVM), improving the rate adaptation, for the production line testing for general digital communication systems and the rate adaptation for multiple-rate communication systems is proposed.